Coupling assembly for coupling a rod to a bone anchoring element, and polyaxial bone anchoring device

ABSTRACT

A coupling assembly is provided for coupling a rod to a bone anchoring element, the coupling assembly ( 4, 4′, 4 ″) comprising
         a receiving part ( 5, 5 ″) having a first end ( 5   a ), a second end ( 5   b ) and a central axis (C) extending through the first end ( 5   a ) and second end ( 5   b ),   a recess ( 52 ) for receiving a rod ( 100 ), and   an accommodation space ( 54 ) for accommodating a head ( 3, 3 ′) of an anchoring element ( 1 ), the accommodation space having an opening ( 55 ) at the second end ( 5   b ) sized so as to permit the insertion of the head ( 3, 3 ′) and a bore ( 51 ) extending from the accommodation space to the first end ( 5   a ),   a retainer element ( 6, 6′, 6 ″) configured to be positioned at least partially in the accommodation space ( 54 ) and sized so as to at least partially encompass an inserted head ( 3, 3 ′);   a spring element ( 7, 7 ″), separate from the retainer element ( 6, 6′, 6 ″), configured to be arranged at least partially in the accommodation space ( 54 ),   wherein the spring element ( 7, 7 ″) is configured to be compressed in the axial direction   and wherein the spring element ( 7, 7 ″) has such a length in the axial direction that it can be engaged by an inserted rod when the retainer element ( 6, 6′, 6 ″) and the spring element are arranged in the accommodation space ( 54 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 61/926,683, filed Jan. 13, 2014, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 14151009.9, filed Jan. 13, 2014, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

The invention relates to a coupling assembly for coupling a rod to a bone anchoring element and to a polyaxial bone anchoring device with such coupling assembly. The coupling assembly comprises a receiving part with a channel for receiving the rod, an accommodation space for accommodating a head of a bone anchoring element and a retainer element for retaining the head of the bond anchoring element in the receiving part. Furthermore, a spring element is provided that is biased in such a manner that the retainer element snaps automatically onto a head of the bone anchoring element when the head is being inserted.

2. Description of Related Art

From US 2013/0150852 A1 a polyaxial bone anchor including a housing, a bone screw, and a retainer for pivotably coupling the head of the bone screw to the housing is known. The retainer is positioned into the bore of the housing and includes a plurality of alternating tabs and slots circumferentially arranged to define a cavity for receiving the head portion of the bone screw therein. The bone anchor further includes a resilient spring means biasing the retainer towards the lower end of the housing. The head portion of the bone screw may apply a force against the retainer opposing and overcoming the biasing force of the resilient spring means. The resilient spring means may be, for example, a wave washer, a helical spring, an elastomeric member etc. or may be circumferential or helical slots formed in the retainer.

U.S. Pat. No. 8,075,603 B2 describes a fastening system consisting of a polyaxial ball and socket joint used in conjunction with a bone screw having threads on one end and a spherical connector on the other hand operating as a pivot point about which a connection assembly moves in a polyaxial fashion. A substantially U-shaped connecting assembly has a lower receptacle that operates as a socket for housing an upper retainer ring and a lower split retainer ring. The socket is receptive to the spherical connector which is inserted through the lower split retainer ring causing a momentary displacement thereof which allows for the positioning of the spherical connector between the upper and lower retainer rings. A resilient component, such as two helical springs, positioned between the upper retainer ring and the connecting assembly permits relative predetermined placement and retention of the spherical connector relative to the connector assembly.

SUMMARY

The known polyaxial bone anchors allow for inserting the spherical head of a bone screw into the receiver by pushing it against the spring force of a resilient member. However, there is still a need for a coupling assembly and a polyaxial bone anchor with such a coupling assembly that is improved with regard to several aspects such as the efficiency and safety of the coupling.

It is an object of the invention to provide a coupling assembly for coupling a rod to a bone anchoring element and a polyaxial bone anchor comprising such a coupling assembly that allows to safely connect the bone anchoring element to the coupling assembly with low insertion force, but high retention force and that requires only a small axial travel to insert the bone anchoring element into the coupling assembly. Also, the coupling assembly shall be easy to manufacture.

The object is solved by a coupling assembly according to claim 1 and by a polyaxial bone anchoring device according to claim 13 or according to claim 15. Further developments are given in the dependent claims.

The coupling assembly includes a receiving part with an accommodation space for accommodating the head of a bone anchoring element and a retainer element configured to be positioned at least partially in the accommodation space. Further, it comprises a spring element in the form of a compression spring that is compressible in an axial direction and that fully extends around the central axis in the circumferential direction and that has an axial length so that it can be engaged by an inserted rod. Preferably, the spring element is a wave spring element. It generates a higher spring force on a given axial length compared to other spring elements. Therefore, the snap-over of the retainer element on the head of the bone anchoring element is facilitated.

The retainer element further may have at least one horizontal slit at its bottom end that contributes to a low insertion force required for inserting the head into the receiving part. Simultaneously, a retention force that holds the head in the receiving part is high compared to the insertion force. Therefore, the bone anchoring element is effectively prevented from being pulled-out from the lower opening. In addition, because of the small insertion path, milling under the head or sticking out of the head from the bone can be avoided.

The coupling assembly may further include a pressure element for exerting pressure onto the head of the bone anchoring element to lock the bone anchoring element in a specific angular position relative to the receiving part. The retainer element may encompass at least partially the pressure element from the outer side thereof so that no additional height of the receiving part for accommodating the retainer element and the pressure element is necessary. Hence, a low profile implant can be provided.

In addition, the receiving part is monolithic and sized such that the retainer element and the spring element as well as the pressure element can be mounted from the top opening thereof.

The pressure element may be held in a position such that the head of the bone anchoring element is held by a frictional force exerted by the pressure element onto the head. The frictional force may be such that pivoting of the head is still possibly by applying a force to overcome the frictional force.

The coupling assembly can be assembled in situ with a bone anchoring element that has been already inserted into a bone or a vertebra.

The head of the bone anchoring element may have at its free end an engagement recess for a driver that comprises a groove shaped and arranged in a spiral-like manner. This allows to transmit high torques onto the bone anchoring element. Furthermore, the free end surface of the anchoring element that comprises the drive recess may have a spherical shape. In an embodiment where the rod presses directly onto the head without using a pressure element, the spherical shape ensures that the necessary contact area between the rod and the head is provided even at large pivot angles of the bone anchoring element.

With the coupling assembly a modular polyaxial bone anchoring device can be provided that may include several bone anchoring elements differing in regard of their length of the shank, their anchoring features of the shank, such as different thread types, thread pitches, different diameter of the shank, and in regard of cannulated or non-cannulated shanks.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become more apparent from the description of various embodiments using the accompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of a first embodiment of a bone anchoring device.

FIG. 2 shows a perspective view of the bone anchoring device of FIG. 1 in an assembled state.

FIG. 3 shows a perspective view from above of a receiving part according to the first embodiment.

FIG. 4 shows a perspective view from the bottom of the receiving part shown in FIG. 3.

FIG. 5 shows a top view of the receiving part shown in FIGS. 3 and 4.

FIG. 6 shows a cross-sectional view of the receiving part shown in FIGS. 3 to 5 along line A-A in FIG. 5.

FIG. 7 shows a perspective view from above of a retainer element according to the first embodiment.

FIG. 8 shows a perspective view from the bottom of the retainer element of FIG. 7.

FIG. 9 shows a top view of the retainer element of FIGS. 7 and 8.

FIG. 10 shows a cross-sectional view of the retainer element of FIGS. 7 to 9 along line B-B in FIG. 9.

FIG. 11 shows a perspective view from above of a spring element according to the first embodiment of the coupling assembly.

FIG. 12 shows a side view of the spring element shown in FIG. 11.

FIG. 13 shows a top view of the spring element of FIGS. 11 and 12.

FIGS. 14 and 15 show cross-sectional views of steps of mounting the retainer element to the receiving part.

FIG. 16 shows a cross-sectional view of the fully assembled coupling assembly according to the first embodiment with the retainer element and the spring element, the section taken in a direction transverse to the rod channel axis.

FIGS. 17 to 18 show cross-sectional views of steps of mounting the coupling assembly of the first embodiment to a bone anchoring element.

FIG. 19 shows a cross-sectional view of the fully assembled polyaxial bone anchoring device with the coupling assembly according to the first embodiment with inserted bone anchoring element and rod, the section taken in a plane perpendicular to the rod axis.

FIG. 20 shows a perspective exploded view of a second embodiment of the polyaxial bone anchoring device with a second embodiment of the coupling assembly.

FIG. 21 shows a perspective view from above of the bone anchoring element according to the second embodiment.

FIG. 22 shows a cross-sectional view of the second embodiment of the polyaxial bone anchoring device wherein the bone anchoring element is inserted into the coupling assembly, the section taken in a plane perpendicular to the rod channel axis.

FIG. 23 shows a cross-sectional view of the fully assembled polyaxial bone anchoring device according to the second embodiment with inserted and fixed rod.

FIG. 24 shows a perspective exploded view of a third embodiment of the polyaxial bone anchoring device with a third embodiment of the coupling assembly.

FIG. 25 shows a cross-sectional view of the receiving part according to the third embodiment of the polyaxial bone anchoring device and the coupling assembly, the section taken in a plane perpendicular to the rod channel axis.

FIG. 26 shows a perspective view from above of the retainer element according to the third embodiment.

FIG. 27 shows a cross-sectional view of the retainer element shown in FIG. 26.

FIG. 28 shows a perspective view from the top of a spring element according to the polyaxial bone anchoring device and coupling assembly of the third embodiment.

FIG. 29 shows a side view of the spring element of FIG. 28.

FIG. 30 shows a perspective view from above of a pressure element of the polyaxial bone anchoring device and coupling assembly according to the third embodiment.

FIG. 31 shows a perspective view from the bottom of the pressure element shown in FIG. 30.

FIG. 32 shows a top view of the pressure element of FIGS. 30 and 31.

FIG. 33 shows a cross-sectional view of the pressure element shown in FIGS. 30 to 32 along line D-D in FIG. 32.

FIGS. 34 to 35 show cross-sectional views of steps of assembling the coupling assembly according to the third embodiment.

FIGS. 36 to 37 show a cross-sectional view of steps of mounting the coupling assembly of the third embodiment to a bone anchoring element.

FIG. 37 shows a cross-sectional view of a fully assembled polyaxial bone anchoring device according to the third embodiment.

FIG. 38 shows a cross-sectional view of the polyaxial bone anchoring device of FIG. 37 with an inserted and fixed rod, wherein the section is in a plane perpendicular to the rod axis.

FIG. 39 shows a cross-sectional view of the polyaxial bone anchoring device according to the third embodiment, wherein the section is in a plane containing the rod axis.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a bone anchoring device according to a first embodiment includes a bone anchoring element 1 in the form of a bone screw having a threaded shank 2 and a head 3. The head 3 has a spherical segment-shaped outer surface portion 3 a, including a greatest outer diameter E of the sphere, and a free end with a recess 3 b for engagement with a screwing-in tool. The bone anchoring device further includes a coupling assembly 4 for coupling a stabilization rod to the bone anchoring element 1. The coupling assembly 4 includes a receiving part 5, a retainer element 6 and a spring element 7 configured to be arranged in the receiving part 5. A pin 8 may be provided for securing the retainer element 6 against rotation in the receiving part 5.

In addition, a locking element 9 in the form of an inner screw is provided for securing the rod 100 in the receiving part 5 and for locking the whole device.

Referring in particular to FIGS. 3 to 6, the receiving part 4 is a monolithic part that is substantially cylindrical and has a first end or top end 5 a, the second end or bottom end 5 b and a central axis of symmetry C passing through the top end 5 a and the bottom end 5 b. A bore 51 is provided that is coaxial to the central axis C. In a first region adjacent to the top end 5 a, the receiving part 5 has a substantially U-shaped recess 52 with a bottom directed towards the bottom end 5 b and two free lateral legs 52 a, 52 b extending towards the top end 5 a. On the legs 52 a, 52 b, an internal thread 53 is provided that cooperates with the locking element 9. The channel formed by the U-shaped recess 52 is sized so as to receive the rod 100 therein for connecting at least two or a plurality of bone anchoring devices. In a region that extends from a distance above the bottom of the U-shaped recess 52 to a distance from the bottom end 5 b the bore 51 has a greater diameter so that an accommodation space 54 is formed that serves for receiving the head 3 of the bone anchoring element 1 and for receiving the retainer element 6 as well as the spring element 7. The accommodation space 54 comprises at its lower end adjacent to the bottom end 5 b of the receiving part a seat portion 54 a for the retainer element 6 that has a smaller diameter than the main portion of the accommodation space 54 and conically tapers toward the bottom end 5 b of the receiving part. By the reduction of the diameter of the accommodation space 54, a shoulder 54 b is provided that may also have the function of a stop for the retainer element 6. An upper edge 54 c between the accommodation space 54 and the bore 51 forms a stop for the spring element 7.

The accommodation space 54 further has an opening 55 at the bottom end 5 b, the inner diameter of which is larger than the greatest outer diameter E of the head 3 of the bone anchoring element 1 so that the head 3 can be inserted from the bottom end 5 b. The edge surrounding the opening 55 comprises a through-hole 56 that extends into the accommodation space 54 and that serves for a press-fit accommodation of the pin 8. The axis of the through-hole may be parallel to the central axis. The through-hole 56 of the receiving part is positioned in a circumferential direction approximately at the center of one of the legs 52 a, 52 b.

Referring more in detail to FIGS. 7 to 10, the retainer element 6 will be explained. The retainer element 6 resembles a cap-like part. It comprises a first end or top end 6 a and an opposite second end or bottom end 6 b. Adjacent to the bottom end 6 b a spherical segment-shaped recess 61 extends into the retainer element 6 with an inner diameter that is such that it matches an outer diameter of the spherical surface portion 3 a of the head 3. The extension of the recess 61 in an axial direction is such that when the head 3 is inserted into the recess 61, the bottom end 6 b extends below the region with the greatest diameter E of the head 3 in a direction toward the threaded shank 2.

A coaxial bore 62 extends from the top end 6 a into the recess 61 to allow access to the head 3 of the anchoring element with a tool. An outer surface of the retainer element 6 has a first portion 63 adjacent to the top end 6 a that is substantially conically shaped and that tapers towards the first end 6 a. The first portion 63 may have two opposite flattened sides 63 a that facilitate gripping the retainer element with a tool (now shown) for assembling the retainer element 6 and the receiving part 5.

Approximately in the region with the largest inner diameter of the recess 61 an outwardly protruding annular edge 64 is formed with a maximum outer diameter that is only slightly smaller than an inner diameter of the accommodation section 54 of the receiving part 5, as shown, for example, in FIG. 16. The outwardly protruding annular edge 64 has an upper side 64 a that is configured to support the spring element 7. Flattened portions 64 b of the annular edge 64 are provided and arranged at substantially 90° with respect to the flattened portions 63 a of the first portion 63. At substantially 90° with respect to the flattened portions 64 b seen in a circumferential direction, a substantially U-shaped recess 65 is provided in the outer surface of the annular edge 64 which extends in an axial direction. The recess 65 is configured to receive the pin 8 therein.

Adjacent to the bottom end 6 b the retainer element has the form of a slit ring 66. The slit ring 66 has a substantially conical outer shape that matches the inner shape of the seat portion 54 a of the accommodation space 54. The inner surface is a portion of the spherical recess 61 so that the inner surface of the slit ring generates a seat for the head 3 to provide a ball and socket joint between the receiving part 5 and the bone anchoring element 1 when the retainer element is mounted to the receiving part 5. As depicted in FIG. 8, the slit ring 66 is formed by a first vertical slit 66 b extending from the bottom end 6 b in a substantially vertical direction. From the vertical slit 66 b two opposite horizontal slits 66 c, 66 d extend circumferentially around the central axis C. The horizontal slits 66 c, 66 d end in widened portions 66 e, 66 f. Between the end portions 66 e, 66 f a connecting portion 66 g is formed that connects the slit ring 66 to the remainder of the retainer element 6. Hence, the retainer element 6 is a monolithic part comprising the slit ring 66. The width of the vertical slit 66 b and of the horizontal slits 66 d, 66 f as well as the width of the connecting portion 66 g may be selected such that a desired flexibility of the slit ring 66 is obtained. By means of the slit ring 66 the retainer element 6 is configured to be expanded and compressed in a radial direction. As can be seen in particular in FIGS. 8 and 10, the vertical slit is at a circumferential position that is substantially the same as the U-shaped recess 65, and the connecting portion 66 g is at the opposite position.

As illustrated in FIG. 16, a total axial length of the retainer element 6 when the retainer element 6 is inserted into the receiving part 5 and when the slit ring 66 is seated in the seat portion 54 a of the receiving part 5 is such that the top end 6 a of the retainer element 6 is approximately at the axial height of the bottom of the U-shaped recess 52 or slightly above. It shall be noted that the retainer element 6 acts also as a pressure element on an inserted head as it not only prevents removal of the head but also exerts pressure to the head from the side and from above.

Referring now to FIGS. 11 to 13, the spring element 7 is formed as a wave spring. It comprises substantially circular turns made out of a flat strip, such as a flat wire. A cross-section of the flat strip may be rectangular. In the embodiment shown, the wave spring has four turns 71, 72, 73, 74. However, it may be contemplated that at least two and less than four or more than four turns are provided. Each turn has a plurality of wave crests 70 a and a plurality of wave troughs 70 b that are alternatingly arranged and form a ring. The turns 71, 72, 73, 74 that are consecutively stacked in an axial direction around the central axis C are connected to each other such that the following turn is offset from a previous turn in a circumferential direction by the length of a crest portion (or a trough portion), as can be seen in particular in FIGS. 11 and 12. This specific shape is known as a crest-to-crest wave spring.

The turns each may be made out of separate flat strips that may be connected to each other, for example by welding. Alternatively, the spring element is made out of a single continuous flat strip that is wound around the central axis so as to form a wave spring shape. Many modifications are possible. The number of turns, i.e. the axial length of the spring element 7 and the spring properties are selected such that a desired compression force and return force is provided by the spring element. The axial length of the spring element is in particular such that, as can be seen in FIGS. 17 to 19, the spring element 7 can be positioned onto the first portion 63 of the retainer element 6 and fills the accommodation space 54 between the upper surface 64 a of the annular edge 64 of the retainer element 6 and the upper edge 54 c of the accommodation space 54. An outer diameter of the spring element 7 is slightly smaller than an inner diameter of the accommodation space 54.

When the spring element 7 is in the accommodation space 54 it may be in a biased condition so that it exerts a pre-load onto the retainer element, but is still further compressible.

An advantage of the wave spring is that compared to a helical compression spring that produces the same compression and return force, the wave spring has a shorter axial length. As a consequence thereof the necessary space for the spring element may be smaller or the travel path for inserting the anchoring element against the spring force can be minimized.

The bone anchoring device as a whole or in parts may be made of a bio-compatible material, such as a bio-compatible metal or a metal alloy, for example titanium, stainless steel, a nickel-titanium alloy, for example nitinol, or of bio-compatible plastic materials, such as, for example, polyetheretherketon (PEEK) or of a bio-compatible ceramic material. In particular, it may be contemplated that the spring element is made of a superelastic nickel-titanium alloy or of beta titanium.

The assembly of the coupling assembly will be explained referring to FIGS. 14 to 16. First, the pin 8 is inserted beforehand in the through-hole 56 of the receiving part 5. Next, the retainer element 6 is inserted into the receiving part 5 from the top and 5 a. It is inserted in a 90° tilted manner such that its central axis is oriented at 90° with respect to the central axis C of the receiving part. Then, as soon as a portion of the bottom end 6 b and the annular edge 64 enters the accommodation space 54, it is tilted again, so that its central axis becomes coaxial with the central axis C of the receiving part 5. The orientation of the retaining element is such that, as depicted in FIG. 15, the U-shaped axial recess 65 provided at the annular edge 64 engages the pin 8.

When the retainer element 6 is finally seated with the slit ring 66 in the seat portion 54 a of the receiving part 5, the spring element 7 is inserted such that it encircles the upper portion 63 of the retainer element and rests on the upper side 64 a of the annular edge 64. The top end 7 a of the spring element abuts against the stop 54 c provided at the upper side of the accommodating space 54. In this condition, the spring element 7 is somewhat compressed, so that it exerts a biasing force onto the retainer element 6 to hold the slit ring 66 in the seat 54 a. When the slit ring 66 is placed in to the seat 54 a, its lower edge 6 b projects slightly out of the lower opening 55 of the receiving part 5. A portion of the spring element 7 is extending above the bottom of the channel 52 of the receiving part 5 so that it can be engaged by the rod.

Next, as depicted in FIGS. 17 and 18, the head of the bone anchoring element 1 is inserted through the lower opening 55 into the receiving part 5. The head 3 first enters into the slit ring 66 through the lower opening 55. As shown in FIG. 18, when the head 3 enters the accommodation space 54, the slit ring 66 is pushed out of its seat 54 a. Simultaneously, the spring element 7 is compressed by the upward movement of the retainer element 6. In addition, the slit ring 66 is expanded when the head 3 further enters into it. The accommodation space 54 provides space for the expansion. As soon as the counterforce exerted by the compressed spring element 7 is greater than the necessary force for expanding the slit ring 66 and for sliding the slit ring 66 over the portion of the largest diameter E of the head 3, the spring force of the compressed spring element 7 causes the slit ring 66 to snap over the head 3, so that its lower edge 6 b easily slides over the region with the largest diameter E. The head 3 can be inserted only to such an extent that it abuts against the upper portion of the spherical recess 61 of the retainer element 6. The retainer element 6 and the spring element 7 cannot escape through the top end 5 a of the receiving part 5 because of the stop 54 c provided in the accommodation space 54.

When the slit ring 66 is below the portion with the greatest diameter E of the head 3, the head 3 can no longer be pulled-out through the lower opening 55.

The slit ring 66 may have a slight undersize with respect to the size of the head 3 such that the slit ring 66 encompasses the head 3 thereby exerting a frictional force onto the head 3. Hence the head 3 may be held by friction within the receiving part and the receiving part can be maintained in a specific angular position with respect to the bone anchoring element 1 before locking of the bone anchoring element. Also the spring force exerted by the biased spring element 7 may contribute to the friction hold of the head 3 in the receiver 5.

As depicted in FIG. 19, in a next step the rod 100 is inserted into the U-shaped recess 52 and the locking element 9 is inserted. When the locking element 9 is tightened, the rod 100 presses onto the top end 6 a of the retaining element and contacts the spring element 7. Final tightening of the locking element 9 locks the slit ring 66 of the retaining element 6 and the head 3 in the receiving part 5. As can be seen in FIG. 19, in the locked condition, the rod 100 compresses the spring element 7.

In use, the bone anchoring element may be inserted into the bone or into a vertebra prior to mounting the coupling assembly. In an alternative manner of use, the bone anchoring element and the coupling assembly are pre-assembled and thereafter inserted into the bone. A plurality of bone anchoring devices can be connected through a stabilization rod.

Referring to FIGS. 20 and 21, a second embodiment of the polyaxial bone anchoring device will be described. The polyaxial bone anchoring device according to the second embodiment differs from the polyaxial bone anchoring according to the first embodiment in the design of the coupling assembly and in particular in the design of the retainer element. All parts that are similar or identical to the previous embodiments are designated with the same reference numerals and the description thereof will not be repeated.

The coupling assembly 4′ comprises a retainer element 6′, which lacks the first portion 61 of the retainer element 6 of the first embodiment. Hence, it consists of an annular portion 64′ with a cylindrical inner surface and of the slit ring 66 that is identical to the slit ring of the first embodiment. With such a design the head of the bone anchoring element can protrude out of the top end 6 a of the retainer element 6′. The size is such that the head protrudes to an extent out of the retainer element 6′ that allows to press directly onto the head with the rod 100.

The bone anchoring element 1′ according to the second embodiment comprises a spherical head 3′ that has also a spherical outer surface at the free upper end. In the embodiment, the head as a whole is spherical. The recess 3 b′ for the tool may have wings that extend in a spiral-like manner from a center point of the upper free end. In the embodiment shown, the recess 3 b′ comprises four wings that are formed by a groove 31 that forms the outer contour of a cross with arms each bent in the same direction. By means of this, the engagement surface of a driver with the recess 3 b′ is enhanced compared to usual polygon or other recesses. Therefore, the loads that can be transferred are higher. Recesses for drivers and corresponding tools with a similar shape are known under the trademark Mortorq®. It may be contemplated that all similar shapes can be used as drive recesses for the head 3′.

As depicted in FIG. 22, when the head has entered the receiving part and the retainer element 6′, its upper portion including the drive recess 3 b′ protrudes out from the retainer element 6′. Also in this embodiment, the head 3 cannot be pulled out through the lower opening 55 once the retainer element 6′ is seated in seat 54 a of the receiving part 5. The spring element 7 projects into the channel 52 for the rod 100. Hence, the rod 100 engages the spring element 7 when it is inserted into the recess 52. When the rod is moved down via the locking element 9, as depicted in FIG. 23, it compresses the spring element 7 which in turn urges the retainer element 6′ into the seat 54 a of the receiving part 5. Finally, the rod 100 presses onto the upper surface of the head 3′, whereby the head 3′ is pressed into the seat provided by the slit ring 66 of the retainer element 6′. Because of the spherical shape of the head 3′, sufficient pressure is also exerted onto the head 3′ in a pivoted state of the bone anchoring element 1′.

The steps of mounting the retainer element 6′ to the receiving part 5 are the same as in the first embodiment.

A third embodiment of the polyaxial bone anchoring device and the coupling assembly will be described with reference to FIGS. 24 to 39. First, as can be seen in FIG. 24, the coupling assembly 4″ includes a receiving part 5″, a retainer element 6″, a spring element 7″ and additionally a pressure element 10 for exerting pressure onto the head 3 of the bone anchoring element 1. It shall be noted that in this embodiment the bone anchoring element 1 is the same as that of the first embodiment. However, it is not limited thereto, but can be another bone anchoring element, for example, that of the second embodiment. Parts and portions of the third embodiment that are identical or similar to that of the first or second embodiment are marked with the same reference numerals and the description thereof will not be repeated.

The receiving part 5″ of the second embodiment is depicted in a cross-sectional view perpendicular to the rod axis in FIG. 25. The receiving part 5″ lacks the through-hole 56 for the pin 8 at the bottom end 5 b. For the purpose of holding a pressure element 10 inside the receiving part 5″ and for preventing rotation of an inserted pressure element 10, the receiving part 5″ includes two transverse pin holes 57 a, 57 b that are positioned in a circumferential direction substantially at the center of the legs 52 a, 52 b. The pin holes 57 a, 57 b are configured to accommodate pins 11 a, 11 b in a press-fit manner. The length of the pins is such that the pins 11 a, 11 b can protrude into the bore 51 of the receiving part to engage the pressure element 10. The axial position of the through-holes 57 a, 57 b is such that as depicted for example in FIG. 36, the pressure element 10 is held by the pins at its upper end to be described more in detail below.

The retainer element 6″ differs from the retainer element of the first and the second embodiment in that there is a portion 61″ adjacent to an upper surface 64 b of the annular outwardly protruding edge 64″ that comprises a plurality of circumferentially arranged upstanding slightly resilient wall portions 63 a that are separated by axial slots so as to provide the elasticity. Each wall portion 63 a has an inwardly protruding edge 63 b. The inwardly protruding edge 63 b is configured to engage a portion of the pressure element 10 so that the pressure element 10 and the retainer element 6″ can be coupled together. An inner and an outer surface of the wall portions 63 a is substantially conical, tapering towards the top end 6 a.

The annular edge 64″ lacks the recess of the retainer element of the first embodiment for receiving the pin, as there is no pin in the third embodiment that holds the retainer element. The slit ring 66 is the same as in the first and second embodiment.

As depicted in FIGS. 28 and 29, the spring element 7″ is also a wave spring, preferably of the type of a crest-to-crest wave spring. It has three turns in the embodiment shown, but the number of turns is not limited to three and could be two or more than three depending on the available space in the accommodation space and of the spring characteristics of the spring element. An inner and an outer diameter of the spring element 7″ is such that the spring element 7″ is configured to be supported on the upper side 64 a of the annular edge 64″ of the retainer element 6″ and extends around the first portion 63″ having the flexible wall portions 63 a.

The pressure element 10 has a first end or top end 10 a and a second end or bottom end 10 b. Adjacent to the top end 10 a there is a substantially cylindrical section with a first outer diameter that is only slightly smaller than the inner diameter of the bore 51 of the receiving part 5″ so that the first portion 101 can be slidingly arranged in the bore 51. Adjacent to the top end 10 a there is a transverse recess 102 with substantially parallel side walls and a substantially V-shaped bottom 102 a that forms a channel for receiving the rod so that the rod axis is perpendicular to the central axis C. The V-shaped bottom 102 a is configured to support rods of different diameter. By the recess 102 two upstanding legs 102 b, 102 c are formed that are provided at their top side with U-shaped recesses 103 a, 103 b, respectively, which are open to the top end 10 a and extend transverse to the channel axis. The recesses 103 a, 103 b are configured to receive a portion of the pins 11 a, 11 b, as can be seen in FIGS. 35 and 36, for example. Each of the legs 102 a, 102 b comprises an outwardly projecting flange 104 a, 104 b that is arranged substantially at the height of the V-shaped bottom 102 a and extends in a circumferential direction around a portion of each leg 102 a, 102 b.

At either end of the bottom of the channel 102 for the rod recesses 105 a, 105 b are provided that allow the insertion of the pressure element 10 when the pins 11 a, 11 b are already mounted into the pin holes 57 a and 57 b.

The pressure element 10 further comprises adjacent to the bottom end 10 b a second portion 106 with a smaller diameter compared to the first portion 101. The second portion 106 is substantially cylindrical and comprises a spherical segment-shaped recess 107 adjacent to the bottom end 10 b. The spherical recess 107 serves for exerting pressure onto the head 3 of the bone anchoring element 1. An outwardly protruding edge 108 is provided at to the bottom end 10 b for engaging the inwardly protruding edge 63 b of the retainer element 6″ to couple the pressure element 10 to the retainer element 6″. The edge 108 has a chamfered lower side for facilitating the insertion into the retainer element 6. Furthermore, the pressure element has a coaxial bore 109 for providing access to the head 3 of the bone anchoring element 1 with a tool (not shown).

The assembly of the coupling assembly 4″ will be explained with reference to FIGS. 34 and 35. Thus, the retainer element 6″ is inserted from the top end 5 a of the receiving part 5″ in a tilted manner and when it has reached the accommodation space 54 it is tilted back and placed onto the bottom of the accommodation space 54 with the slit ring 66 being in the seat 54 a.

When the retainer element 6″ has been inserted into the receiving part 5″, the spring element 7″ is also inserted from the top end 5 a of the receiving part 5″ and placed on top of the retainer element 6″ so that it rests on the annular edge 64″.

Next, the pressure element 10 is inserted into the receiving part from the top end 5 a. As the pins 11 a, 11 b are already received in the pin holes 57 a, 57 b of the receiving part, the pressure element is inserted in a 90° rotated position so that its channel 102 is oriented at 90° with respect to the U-shaped recess 52 of the receiving part 5″. In this configuration, the pins 11 a, 11 b can pass through the slits 105 a, 105 b. When the pressure element 10 enters the upper portion 63″ of the retainer element 6″ the flexible wall portions 63 a are deflected slightly outward to allow the insertion of the bottom end 10 b of the pressure element 10 until the inwardly protruding edge 63 b of the retainer element snaps behind the outward edge 108 of the lower portion of the pressure element 10. Once the flanges 104 a, 104 b have entered the accommodation space 54, the pressure element can be rotated so that the channel 102 for receiving the rod of the pressure element 10 and the U-shaped recess 52 of the receiving part 5″ are aligned. Rotation is only possible, when the pressure element is with its top end 10 a below the pins 11 a, 11 b.

After alignment of the pressure element, the pins 11 a, 11 b are received in the U-shaped recesses 103 a, 103 b, respectively, of the pressure element 10. The bottom of the U-shaped recesses 103 a, 103 b forms an abutment for the pressure element 10 and prevents escaping of the pressure element 10 through the top end 5 a. In the preassembled state of the coupling assembly 4″, as depicted in FIG. 35, the spring element 7″ is slightly pre-compressed.

The mounting of the coupling assembly 4″ to the bone anchoring element 1 is shown in FIGS. 36 and 37. As in the first embodiment, the head 3 enters through the lower opening 55 into the retainer element 6″ and therefore into the accommodation space 54. The slit ring 66 is pushed out of its seat and the retainer element 6″ is moved upward whereby the contacting surfaces of the outer rim 108 of the pressure element 10 and of the inwardly protruding edge 63 b of the retainer element 6″ disengage. During upward movement of the retainer element 6″, the spring element 7″ is compressed whereby it abuts against the lower side of the flanges 104 a, 104 b of the pressure element 10. The upward movement is limited by the pins 11 a, 11 b.

During the insertion of the head 3 into the retainer element 6″ the slit ring 66 expands. As soon as the counterforce exerted by the compressed spring element 7″ is greater than the necessary force for expanding the slit ring 66 and for sliding this slit ring 66 over the portion of the largest diameter E of the head 3, the spring force of the compressed spring element 7″ causes the slit ring 66 to snap over the head 3 so that its lower edge 6 b easily slides over the region with the largest diameter E. The head 3 can be inserted until it abuts against the spherical recess 107 of the pressure element 10. When the spring force shifts the retainer element 6″ downward, the inwardly protruding edge 63 b and the outward rim 108 engage again.

The pre-stress exerted by the spring element 7″ and/or a slight undersize of the slit ring 66 compared to the size of the head 3 leads to a frictional hold of the head 3 in the receiving part before finally locking the head. Also, when the slit ring 66 enters the seat 54 a again a removal of the head 3 is no longer possible.

Finally, the rod 100 is inserted and pressed down by tightening the locking element 9. The pressure exerted by the pressure element 10 onto the head 3 further presses the slit ring 66 into the seat 54 a and final tightening locks the head and the retainer element in the receiving part 5″.

Further modifications of the embodiments may be contemplated. For example, for the bone anchoring element, various different kinds of anchoring elements can be used and combined with the receiving part. The anchoring elements may be, for example, screws with different lengths, screws with different diameters, cannulated screws, screws with different thread forms, nails, hooks, etc. For some anchoring elements, the head and the shank may also be separate parts that can be connected to each other.

Other possible modifications of the receiving part may include, for example, instead of the U-shaped recess being perpendicular to the central axis, a recess for the rod may be inclined, open to the side, or in the form of a closed channel. Other kinds of locking devices including outer nuts, outer caps, bayonet locking devices, or others are also possible. In particular, a two-part locking device that includes a first locking element that exerts pressure via the pressure element onto the head and the second locking element that exerts pressure only onto the rod to lock the head and the rod independently, may also be used. In some embodiments, the inner surface portion of the pressure member that contacts the head, may not necessarily be spherically-shaped. The inner surface portion may have any other shape that is suitable to exert pressure onto the head. Also, the design of the pressure element can be different and is not limited to the specific design shown in the third embodiment.

Instead of the pin for retaining the pressure element and for aligning the pressure element with respect to the channel for receiving the rod of the receiving part, other retaining mechanisms can be used.

The head of the bone anchoring element needs not to be rotationally symmetric. For example, the head may have two opposite flat surface portions between two spherically-shaped outer surface portions so as to achieve pivoting in only one plane.

Instead of the slit ring, a plurality of vertically extending slits or a combination of substantially vertically and substantially horizontally extending slits may be provided.

The seat for the slit ring and the outer surface of the slit ring needs not to be conical. Any shape that provides for a safe holding of the slit ring may be contemplated, such as, for example, a spherical shape.

For the spring element other spring elements can be used. For example, a helical spring encircling the central axis may be used. In addition, other spring elements like elastomeric cushions may be contemplated.

It shall be noted that parts of the different embodiments described may be mixed among each other or exchanged so that a variety of further embodiments are generated.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. 

1. A coupling assembly for coupling a rod to a bone anchoring element, the coupling assembly comprising a receiving part having a first end, a second end and a central axis extending through the first end and second end, a recess for receiving a rod, and an accommodation space for accommodating a head of an anchoring element, the accommodation space having an opening at the second end sized so as to permit the insertion of the head and a bore extending from the accommodation space to the first end, a retainer element configured to be positioned at least partially in the accommodation space and sized so as to at least partially encompass an inserted head; a spring element, separate from the retainer element, configured to be arranged at least partially in the accommodation space, wherein the spring element is configured to be compressed in the axial direction and wherein the spring element has such a length in the axial direction that it can be engaged by an inserted rod when the retainer element and the spring element are arranged in the accommodation space.
 2. The coupling assembly of claim 1, wherein the spring element extends fully around the central axis.
 3. The coupling assembly of claim 1, wherein the spring element includes at least one flat strip forming a plurality of alternating successive waves each having a wave crest portion and a wave trough portion and wherein the spring element comprises at least two turns of such successive waves that extend around the central axis.
 4. The coupling assembly of claim 1, wherein the spring element includes a crest-to-crest circular wave spring.
 5. The coupling assembly of claim 1, wherein the retainer element comprises a support surface for supporting the spring element and wherein the axial length of the spring element is such that when the spring element is not compressed and resting on the support surface, it extends in axial direction to a position higher than the bottom of an inserted rod.
 6. The coupling assembly of claim 1, wherein the retainer element is secured in the receiving part against rotation.
 7. The coupling assembly of claim 1, wherein the retainer element comprises a first end and a second end and a first slit spaced apart from the second end that extends at least partially around the central axis, and a second slit that extends from the second end of the retainer element into the first slit, and wherein the first slit extends away from the second slit and is longer than the second slit.
 8. The coupling assembly of claim 7, wherein the retainer element comprises a third slit extending partially around the central axis so that a slit ring is formed at the second end of the retainer element.
 9. The coupling assembly of claim 1, further comprising a pressure element confirmed to be positioned at least partially in the accommodation space and being configured to exert pressure onto the head.
 10. The coupling assembly of claim 9, wherein a stop is provided for the pressure element that prevents upward movement of the pressure element once it has been inserted into the receiving part.
 11. The coupling assembly of claim 1, wherein the bore in the receiving part is sized such that the retainer element is insertable into the receiving part from the first end.
 12. The coupling assembly of claim 9, wherein the bore in the receiving part is sized such that the pressure element is insertable into the receiving part from the first end.
 13. A polyaxial bone anchoring device comprising a bone anchoring element having shank for anchoring to bone and a head, and a coupling assembly according to claim
 1. 14. A polyaxial bone anchoring device of claim 13, wherein the head of the bone anchoring element comprises a free end surface with an engagement recess for a driver wherein the recess comprises a plurality of wing portions that extend in a spiral-like manner from a center point of the free end surface.
 15. A polyaxial bone anchoring device comprising a receiving part having a first end, a second end and a central axis extending through the first end and second end, a recess for receiving a rod, and an accommodation space for accommodating a head of an anchoring element, the accommodation space having an opening at the second end sized so as to permit the insertion of the head and a bore extending from the accommodation space to the first end, a retainer element configured to be positioned at least partially in the accommodation space and sized so as to at least partially encompass an inserted head, wherein the retainer element is configured to lock an inserted head with respect of the receiving part; and further including a bone anchoring element comprising a free end surface with an engagement recess for a driver wherein the recess comprises a plurality of wing portions that extend in a spiral-like manner from a center point of the free end surface.
 16. The polyaxial bone anchoring device of claim 15, wherein the retainer element is shaped such that the head can partially extend therethrough in order to be directly engaged by an inserted rod. 